A focus detection process by using the phase difference detection method causes light fluxes passing through mutually different pupil areas of an image taking optical system (that is, pupil-divided light fluxes) to form paired object images, photoelectrically converts the paired object images to produce paired image signals, and then calculates a phase difference between the paired image signals. The focus detection process further calculates a focus state (defocus amount) of the image taking optical system based on the phase difference.
Such phase difference detection method focus detection uses a photoelectric conversion element in a focus detection unit separate from an image sensor that photoelectrically converts an object image formed by a light flux from the image taking optical system or partial pixels of the image sensor.
Japanese Patent Laid-Open No. 2009-003122 discloses an image capturing apparatus that provides two focus detection pixel groups in an image sensor, that performs pupil-division of a light flux from an image taking optical system into two light fluxes by off-centering aperture portions of one and the other of the two focus detection pixel groups to mutually opposite sides, and that photoelectrically converts paired object images formed by the two light fluxes by the two focus detection pixel groups.
There is a problem in the phase difference detection method focus detection that vignetting of the light flux traveling toward the focus detection unit or the image sensor provided with the focus detection pixel groups decreases a degree of coincidence of the paired object image signals, which results in deterioration of focus detection accuracy. A state of the vignetting is changed with states of the image taking optical system such as a zoom state, a focus state or an aperture stop state and with an image height.
Japanese Patent Laid-Open No. 2010-117679 discloses an image capturing apparatus that restores paired image signals according to a light flux vignetting state by a filter process to increase a degree of coincidence of the paired image signals, and thereby improves focus detection accuracy.